In a province as sunny as
Alberta, wearing sunscreen
in the spring and summer
is important. What’s not
as well known is the math
behind sun protection factor
(SPF) numbers. Many of us
assume “more is more” and
use a higher SPF sunscreen
believing we’re getting
stronger protection, but that’s
not quite how it works.

The SPF of a sunscreen
doesn’t refer to its strength,
but rather to the amount of
time it will (theoretically)
protect you from UV rays.

(See sidebar.) Likewise, if you

Sunscreen protectionis about time, not strengthKnowing the math behind numbers

Myth-busting

apply sunscreen with SPF 30,
you should max out your sun
exposure at 300 minutes.

While a higher SPF will have
you out in the sun longer, it
doesn’t mean SPF 30 is twice
as strong as SPF 15. Sunscreen
with SPF 15 blocks 93 per
cent of UVB rays, while SPF
30 blocks 97 per cent—that’s
a small difference in their
effectiveness.

In the end, it is best not tobother getting too hung upon the math. Patrick Curley,a program co-ordinator forAlberta Health Services’Environment Unit, warnsthose calculations don’talways reflect real life. SPF isdetermined in a lab setting,says Curley. “In reality, thereare so many other factorsrelated to sun exposure thatare not controlled.”The damage from UVexposure is a result of both thelength of time you are outsideand the UV index, which ranksthe strength of ultraviolet rayson a scale of zero to 11.

“The UV index is affected
by time of day, geographical
location, season and more,”
says Curley. When the UV
level reaches three or higher,
extra sun protection is needed.

How you spend your time
in the sun is also a factor. “SPF
15 is fine as a minimum if you
are running errands or outside
for brief periods of time, but
if you are doing an outside
activity or working outdoors,
at minimum you should apply
SPF 30,” says Curley.